I guess the bit of a union that's not wanted is the collective bargaining bit, which probably seems unappealing or pointless while web developers are generally in demand.

Sorry, but collective bargaining is a real need, even if wages are high.

As programmers, we should have:

    * readily available legal support when faced with non-paying clients.
    * recourse against clients and employers who use reputation threats to justify 
      nonpayment or bad behavior (i.e. "if you don't accept our unfunded scope creep, 
      we'll call other clients.")
    * (for employees) representation when in trouble with management. The right to 
      have a representative *in the room* (mostly, because if you negotiate on your
      own behalf, you're at risk of getting emotional and saying something
      stupid) when dealing with management or HR.
    * defense against negative references and back-channel reference checks in 
      general. 
    * transparency in compensation.
    * negotiation of terms that are embarrassing for an individual to
      formally address, such as severance, health accommodations
      and workplace privacy. 

These are things that "collective bargaining" can get. Hollywood actors have a union, even though they're clearly in high demand. In a celebrity economy, you need unions and agents just to deal with the reputation-management issues that quickly become a full-time job in their own right (and that are typically ill-handled by creative specialists like actors or computer programmers). As white-collar employment becomes increasingly reputation-driven, we need this infrastructure yesterday.

Let's say that you find out that an ex-boss from 4 years ago is bad-mouthing you and costing you gigs. If you hire a nasty fucker like me, I can make him shut the fuck up. I'd find a plausible health problem, either one you have and don't mind sharing, or one that would work. Then I'd ask you to communicate to him that you had disclosed that health problem during your employment, his error if he forgot, and that negative communication about you or your performance will be judged discriminatory, so it's best not to say anything at all. Then I'd call that reference for you, about a month later, getting everything on tape and if he was still giving you problems, I'd refer you to a lawyer (but, unlike a union, I can't afford to pay for that). Chances are, you wouldn't know how to handle that. Most people don't. A competent union will.